In a typical neurological monitoring environment, one or more sets of electrodes are attached to a patient's body. In turn, the electrodes are electrically connected to one or more pieces of monitoring equipment through signal wires. Depending on the placement of the electrodes, the monitoring equipment can be utilized to monitor various physiological signals to conduct patient tests, such as an electrocardiogram (“ECG”), an electroencephalogram (“EEG”), an electromyogram (“EMG”), and the like. Often, the physiological signals obtained from a patient may be of a low amplitude. For example, a typical physiological signal may have an amplitude as small as 0.2 μV. Accordingly, the low amplitude signals are highly susceptible to interference from environmental electrical/magnetic sources to the extent that the ability for the monitoring system to function properly is affected.
One skilled in the relevant art will appreciate that neurophysiological monitoring in surgical environments can present significant amounts of environmental interference. Generally described, a typical surgical environment can generate an electrically hostile environment due to the wide variety of electrical devices present in the surgical environment and their relative proximity to the patient and electrode wires. Additionally, hospital sterility regulations often require that some monitoring devices be outside of the direct surgical environment, thereby requiring a long set of electrode wires. Accordingly, the long length of the wires can increase the susceptibility of the low amplitude signals to interference generated by the various electrical devices along the path of the long length electrode wires. Additionally, long length wires can also be more susceptible to environmental electromagnetic interference. Accordingly, most remote monitoring systems attempt to mitigate interference caused by electrical and magnetic sources to better improve the accuracy of the system.
One attempt to mitigate the amount of interference relates to the shielding of some portion of the electrode wires with a braided copper shield. Although copper shielding may reduce some portion of electrical interference, a copper braided shield is generally ineffective against magnetic interference. More specifically, in a surgical environment, copper shielding is generally ineffective in reducing magnetic interference caused by cathode ray tube (“CRT”) displays, drills, cutters, microscope lights, blood warmers, and anesthesia machines.
Another attempt to mitigate the amount of environmental interference relates to the use of a differential amplifier in the monitoring system. One skilled in the relevant art will appreciate that a differential amplifier will reject a common interfering signal received from a set of electrode inputs according to a factor known as a common mode rejection ratio (“CMRR”). For example, a high CMRR can result in smaller amplitude interference, which is especially effective for reducing low frequency electrostatic interference. However, because a CMRR is finite, the effectiveness of a differential amplifier may be reduced for lower amplitude signals, such as physiological signals, especially for environments, such as a surgical environment, that experience higher amounts of interference. Additionally, a differential amplifier's CMRR will generally be reduced with an increase in frequency. Thus, in surgical environments, the use of differential amplifiers alone is generally ineffective in mitigating the effects of electrical interference upon the physiological signals.
Another attempt to mitigate environmental interference relates to utilizing a differential amplifier in combination with physically twisting the signal wires together to cancel out the effects of low frequency magnetic interference. One skilled in the relevant art will appreciate that in a set of twisted wires (e.g., electrodes), the induced current in one twist tends to cancel out the same induced current generated in an adjacent twist. Generally described, the effectiveness of the twisting of two wires can be limited to reducing interference in environments in which an essentially uniform magnetic field strength is present. For example, in a surgical environment, magnetic interference may be caused by a near-field source that generates a non-uniform field from twist to twist. Additionally, if the spatial geometry of the twists is not uniform throughout the entire cable, the effectiveness is further reduced. Referring again to a surgical environment, the spatial geometry of the wires is often at issue because of the often great lengths of wire required. Thus, twisting signal wires is not sufficient to adequately mitigate interference associated with physiological monitoring.
Yet another attempt to mitigate environmental interference relates to the use of a ferrous metal hose to shield the wires. Although a shield including a ferrous hose can reduce low frequency magnetic interference, the application of a ferrous hose shielded cable can become impractical in several environments. Referring again to a surgical environment, ferrous metal hose shielded cables are generally not suited to be handled and easily manipulated, as they are bulky, semi-rigid, and weighty. Moreover, a ferrous hose shielded cable is not well suited for standard mass production cable manufacturing techniques by requiring the addition of insulation to the outside of the hose. Accordingly, this increases the overall cost of monitoring systems.
Thus, there is a need for a system and device capable of mitigating environmental interference in high interference environments.